FIG. 1 illustrates a typical wireless telecommunications system, such as employed by a cellular telephone network, comprising a plurality of base stations, depicted by base stations 203-1 through 203-5. Wireless terminals, depicted by terminals 201-1 through 201-3, communicate with a base station which is located in the same pre-determined geographic area, or cell, as itself. For instance, wireless terminals 201-1 and 201-2 are located in cell A, therefore, under normal circumstances, communicate with base station 203-1, which is located in and services cell A.
As is well-known in the art, in order for wireless terminal 201-1 to communicate, it sends a signal via radio waves to base station 203-1; base station 203-1 relays the received signal to a switching center (not shown); and the switching center, according to instructions supplied as part of the signal, relays the signal elsewhere. If the desired destination of the signal is another wireless terminal, then the switching center relays the signal to a base station located in the same cell as the wireless terminal intended to receive the signal, and the base station transmits the signal via radio waves to the wireless terminal.
One problem experienced by cellular phone systems, or any wireless transmission system, is that of unintentionally terminated, or "dropped", calls. A large fraction of dropped calls are caused by weather patterns. This is illustrated in FIG. 2, which is a graph that shows a percentage of cellular calls that were dropped between Jan. 1, 1996 and approximately Jun. 1, 1997. As shown, a significantly larger percentage of calls are dropped during the summer months than are dropped during other times of the year. It has been determined that, while several factors are responsible for the increased percentage of dropped calls, the largest contributing factor is an atmospheric condition known as tropospheric, or atmospheric, ducting (hereinafter referred to as "ducting").
Generally, ducting is a radio wave propagation mechanism that occurs when, due to weather conditions in the atmosphere, the curvature of the path of a wireless radio wave transmission exceeds the curvature of the earth's surface. As a result, the radio wave is received back at the earth's surface far beyond the distance which would normally be expected. The atmospheric conditions that cause the excessive curvature of the radio wave have distinctive refractive properties, as discussed below. Refraction is the deflection of an energy wave from a straight path, caused when the energy wave passes from one medium through which the energy wave travels at a first velocity, into another medium through which the energy wave travels at a different velocity. Refraction is well-known in the art, as are the refractive properties that result in ducting. These are described in H. Hitney, Propagation Modelling and Decision Aids for Communications Radar and Navigation Systems, AGARD Lecture Series (September 1994), which is incorporated herein as fully as if set forth in its entirety, and are discussed below.
Briefly, as shown in FIG. 3, when a radio wave is transmitted from transmission point 101, it moves through the atmosphere in a straight line, shown as path 102a, until it reaches an atmospheric layer having a refractive index, n. The refractive index n is defined as n=c/v, wherein c and v are the speeds of an energy wave (such as a wireless transmission) in a vacuum and in the atmosphere, respectively. In this case, the radio wave enters atmospheric layer 103, which has a different refractive index, designated as n.sub.0, which is different from the refractive index of the atmospheric layer through which the radio wave was originally transmitted. The different refractive index causes the radio wave to travel at a different velocity and hence causes the path of the radio wave to bend. As a result, the radio wave travels in a new path, shown as path 102b. When the radio wave enters yet another atmospheric layer 104, which has a refractive index different from n.sub.0, the path of the radio wave bends again so as to travel in path 102c. As a result, radio waves may travel in various patterns as governed by refractive conditions of the atmosphere based on the refractivity of various atmospheric layers.
Refractivity N is a measure of a medium's (e.g.--an atmospheric layer) power to refract an energy wave, and is defined as N=(n-1).times.10.sup.6. Refractivity is related to atmospheric parameters by the formula N=77.6[P/T+4810e/T.sup.2 ], wherein P is pressure (hPa), T is temperature (K), and e is partial water vapor pressure. FIG. 4 is a diagram showing four types of refractive conditions that can be experienced by a wireless transmission: trapping, superrefractive, normal and subrefractive. Table 1 indicates the refractivity ranges that result in each refractive condition. As can be seen from Table 1 and FIG. 4, ducting occurs when a trapping refractive condition exists, i.e.--when a radio wave encounters an atmospheric condition having a refractivity wherein Nper kilometer of height &lt;-157. Thus, when an atmosphere has pressure, temperature, and partial water vapor pressure conditions resulting in the refractivity of the atmospheric layer being less than -157 N/km, radio waves passing through the atmospheric layer are trapped and propagate far beyond their expected range.
When this excessive propogation occurs, a wireless transmission (such as a call from a cellular telephone) intended to be transmitted to a nearby receiver station (such as a base station located in the cell in which the cellular phone caller is located) can instead be transmitted to a receiver station much farther away. In some applications, such as radar, this situation may be advantageous. However, in a cellular phone system, it may result in calls being setup at abnormally long distances, such as a 911 emergency call inadvertently being transmitted to a location a far distance from the location of the call, or the imposition of unnecessary long distance charges. More significantly from the standpoint of dropped calls, this situation may result in wireless signals from distant cells causing interference levels in a local cell to increase undesirably. It is this increased interference from distant cells which contributes to the increased number of dropped calls, particularly during the summer months, as wireless transmissions handled by the base station of a local cell are interrupted by wireless transmissions originating from locations at a far distance from the base station.
Thus, there is a need for a system and method that reduces the effect of atmospheric ducting on wireless transmissions.